Method for forming powdered metal into sintered hollow bodies



July 12, 1966 O D. D. RAPPRICH ET AL 3,260,596

METHOD FOR FORMING POWDERED METAL INTO SINTERED HOLLOW BODIES Original Filed July 31. 1962 5 Sheets-Sheet 1 112%] ENTOR.

BY wiw I I ZLZQ-ATTORNEK July 12, 1966 D. D. RAPPRICH ETAL 3,260,596

METHOD FOR FORMING POWDERED METAL INTO SINTERED HOLLOW BODIES Original Filed July 31, 1962 5 Sheets-Sheet 2 ATTORNEY.

July 12, 1966 INVENTOR.

BY W Z 3* ATTORNEY July 12, 1966 D. D. RAPPRICH ET AL 3,260,596

METHOD FOR FORMING POWDERED METAL INTO SINTERED HOLLOW BODIES Sriginal Filed July 31, 1962 5 Sheets-Sheet 5 IN VENTOR.

7 m6; ATTOENE).

United States Patent 3,260,596 METHOD FOR FORMING POWDERED METAL INTO SINTERED HOLLOW BODIES David D. Rapprich, Vermilion, and Francis J. Holewinski,

Toledo, Ohio, assignors to The Brush Beryllium Company, Cleveland, Ohio, a corporation of Ohio Original application July 31, 1962, Ser. No. 213,709, now Patent No. 3,189,942, dated June 22, 1965. Divided and this application Apr. 12, 1965, Ser. No. 461,217 2 Claims. (Cl. 75-226) This application is a division of our pending application, Serial No. 213,709, filed July 31, 1962, now US. Patent No. 3,189,942, and titled Method and Apparatus for Forming Powdered Metal Into Sintered Hollow Bodies, and which is directed to the apparatus, the pres ent application being directed to the method.

Thi invention relates to a method for forming metal powder into sintered hollow bodies, such as sleeve-shaped and cup-shaped bodies, and the like, and more particularly for forming metal powder, in the absence of lubricants, into sintered hollow bodies each of which, at the completion of the sintering operation and before machining, not only has an axial length more than twice the wall thickness, but also is of substantially theoretical density throughout.

While the invention is useful for forming such bodies from any one of a number of metals which differ from each other widely in sintering temperatures and melting points, it is particularly useful in connection with beryllium in that beryllium powder can be formed into such bodies thereby and it cannot be so formed by the prior sintering procedures. Accordingly hereinafter, the method will be described generally as applied to the formation of such bodies from beryllium powder, except where otherwise specifically noted, its use in connection with other metals being readily apparent from the illustrative example.

A common prior technique employed in forming sintered bodies from metal powder is similar to that disclosed in US. Patent No. 2,885,287, issued May 5, 1959, wherein pressure is applied to the powder in a die cavity in a direction endwise of the cavity by means of a ram operating a punch or male die. Such technique, however, creates pressure gradients within the sintering charge of powder in the die cavity or mold assembly during the pressure sintering operation. These pressure gradients result mainly from friction between metal powder and the die wall. Due to this friction the pressure imposed on the powdered particles progressively decreases as the distance from the ram increases, whereby the particles farthest from the ram receive the least pressure. The pressure gradients, in turn, cause longitudinal density gradients within the sintered body with the result that a sintered body of non-uniform density is produced.

In attempting to reduce or overcome this non-uniformity in density, excessive ram pressures have been employed. The use of these excessive pressures generally results in excessive seizure of the powder on the die walls with resultant binding or freezing. Once freezing occurs, increased ram pressure not only does not overcome the frictional effects, but augments them to such a degree that no substantial further compaction can be effected.

A prior technique employed to form sintered hollow bodies of substantially theoretical density is disclosed in US. Patent No. 2,398,227, issued April 9, 1946. Such a technique is limited to bodies in which the length is less than twice the wall thickness, unless the lubricity of the powdered material is increased. Usually, if the lubricity is to be increased, powdered graphite or other lubricating material is added to the metal powder, or the die wall is lubricated; both are well known in the art. The lubriice cants are undesirable because they contaminate the sintered bodies. Even with such lubricants the length to wall thickness ratio can be increased but slightly.

The primary object of this invention is to provide a new and improved method for efiiciently forming metal powder, in the absence of lubricants, into hollow sintered sleeve or cup shaped bodies not only having a length greater than twice the wall thickness but also having substantially the theoretical density of the metal employed.

Other objects and advantages will become apparent from the following description wherein reference is made to the drawings, in which:

FIG. 1 is a front elevation of a furnace and die assemblage embodying the principles of the present invention, part thereof being shown in section for clearness in illustration;

FIG. 2 is an enlarged vertical axial sectional view of the die assemblage of FIG. 1, showing the assemblage with the parts in starting position, and with a charge of metal powder in the die cavity at the beginning of the forming operation and is taken on line 22 of FIG. 1;

FIG. 3 is an enlarged View similar to FIG. 2 showing the die assemblage and metal at the end of the sintering operation;

FIG. 4 is a top plan view of a retainer in the form of a sealing annulus used in the present invention and operable for assuring escape of gases from the powder charge and die cavity while preventing the conveyance of metal powder from the die cavity by escaping gases;

FIG. 5 is a cross sectional view taken on the line 5-5 in FIG. 4;

FIG 6 is a horizontal sectional view taken on the line 6-6 in FIG. 3; and

FIGS. 7 and 8 are views similar to FIGS. 2 and 3, illustrating a modification of the die assemblage.

Referring first to FIGS. 1 and 2, an apparatus for practising the present method is illustrated. As there illustrated, the apparatus comprises a conventional vacuum sintering furnace 1 in which the temperature and degree of vacuum can be accurately controlled.

A ram 2, operated by a reversible hydraulic piston and cylinder assemblage 3, is provided. The pressure from the assemblage 3 is transmitted to the ram through a water cooled pressure transmitting member 4 which extends into the furnace from the outside. Hydraulic pressure may be supplied to the assemblage 3 by a conven tional motor driven pump P, through a reversing valve R, and a pressure control valve C, or by any available controlled pressure source.

The specific details of the ram and its controls, and of the furnace, its heating means, and vacuum pumps, and the like, are not a part of the present invention. It is necessary only that the furnace be capable of heating the die assemblage and the charge of metal powder in the die cavity to sintering temperature while maintaining the charge under vacuum, and that the ram be capable of supplying controlled mechanical pressure up to the maximum required for compacting the powder to the degree desired.

The die assemblage shown for purposes of illustration is for forming elongated sleeves of circular cross section, and may comprise a thick base plate 5 having a flat bottom face by which it is supported on a bed 6 in proper position for cooperation of the ram 2 therewith. The die assemblage comprises a first part in the form of a mandrel or male die 7, and a cooperating second part, in the form of a female die 8. The die 8 is initially supported in fixed axial relation to the mandrel 7, as will later be described, for defining therewith an annular die cavity 9, which is closed at the bottom and open at the top. The assemblage also includes a third part in the form of an annular plunger 10 which is arranged to be forced into the open upper end of the die cavity 9 for initially compacting the material in the cavity while the cavity remains unchanged in shape, and preparatory to final forming of the charge of powder.

If desired, the mandrel 7 may be formed of a plurality of coaxial annular plates 11, 12, 13, and 14, respectively. These plates preferably have the same internal diameter and are bolted together in the coaxial relation shown, by suitable longitudinally extending bolts 7a, and are positioned on the base plate in proper alignment with the ram 2.

In the particular form illustrated, the rings 11 and 12 have frusto-conical outer surfaces with their larger bases facing downwardly so that their outer peripheral surfaces form one continuous surface from the upper face of the plate 5 to the lower face of the plate 13. The plate 13 has a cylindrical external surface of the same diameter as the smaller diameter of the external surface of the plate 12. The top plate 14 has a cylindrical outer surface which is preferably slightly larger than the external diameter of the plate 13, for purposes later to be described.

The female die 8 may'comprise a series of coaxial annular plates, indicated at 15, 16, 17 and 18, respectively, these plates being arranged in coaxial relation and bolted together by bolts 8a so as to form a substantially unitary structure. The inner periphery of the plate is preferably cylindrical and spaced radially outwardly from the adjacent outer peripheral surface of the mandrel. The inner peripheral surfaces of the plates 16, 17, and 18 define a surface which, part way of its length from the bottom upward-1y, is cylindrical, as indicated at 19, then frusto-conical with the larger base downwardly, as indicated at 20, and then again cylindrical, as indicated at 21.

If desired, the mandrel and the female die each may be formed as a unitary structure. However, due to the weight involved, it is preferable for ease in manipulation in assembly and removal of the sintered body, that each be formed as a series of annular plates bolted together firmly as described.

The base plate 5 is provided with an upwardly facing annular radially extending shoulder 24 which is offset downwardly axially from the upper face of the plate 5. Supported removably on the shoulder 24 is an annular compression ring 25 which forms a removable part of the mandrel. The ring 25 has an inner annular face 26 spaced radially outwardly from an exterior annular face 27 on the plate 5 between the upper face and shoulder 24 of the plate 5.

A charge of metal powder 28 to be sintered into an elongated shell is disposed in a container 30 which, in turn, is disposed in the die cavity 9 between the external surface of the mandrel 7 and the interior peripheral surface of the female die 8.

The container 30 preferably is formed of mild steel sheet material and has an outer peripheral wall 31 and an inner peripheral wall 32. These walls are parallel, respectively, to the inner peripheral wall of the female die 8 and the outer peripheral wall of the mandrel 7 throughout the greater portion of their extent endwise of the container. The upper end of the container is open, as indicated at 33, for receiving the annular plunger 10 which effects initial compaction of the powder preparatory to movement of the female die 8 downwardly relative to the mandrel 7 during the final compacting and sintering operation.

For purposes of permitting the escape from the container of gas and air entrapped in, or evolved from, the metal powder, and driven out of the charge due to pressure resulting from closure of the dies or lowering of the plunger 10, while preventing the conveyance of powder out of the container by the gases, filter means 34 are provided. The filter means 34 are disposed in the open end 33 of the container, and are movable downwardly by, and with, the plunger 10. The plunger 10 fits into the upper end of the container with slight radial clearance, and the filter means prevent the escape of powder with the gases and air which are discharged from the upper end of the container between the periphery of the plunger 10 and the container walls.

A suitable filter means is illustrated in FIGS. 4 and 5 and comprises an upper annular bearing plate 35 and a lower annular bearing plate 36, between which an an nular filter 37 of material, such as porous mineral wool fiber capable of withstanding temperatures in excess of 1000 C., is secured in coaxial relation by conventional bolts extending through the plates 35 and 36. An example of this material is one sold under the trade name CERA- FELT, by the Johns-Manville Company, and which has a high aluminum silicate content and is capable of withstanding temperatures up to 1100 C.

As illustrated, the plates 35 and 36 are preferably coaxial and of slightly less external and slightly greater internal diameter than the external and internal diameter of the cavity in the upper end of the container. The filter 37, however, has a somewhat smaller internal diameter and greater external diameter than the cavity so that it snugly fits the interior peripheral wall surfaces of the container walls 31 and 32 at the upper end of the container 30.

In the forming of the metal powder, particularly beryllium powder, into such bodies extreme care must be taken to prevent the escape of the powder from the bottom of the die cavity during initial compaction and during sintering. For this purpose it is desirable that the bottom of the cavity, which when the container 30 is used, is the bottom of the container, is sealed to prevent the escape of metal powder between the lower inner periphery of the female die, as indicated at 38, and the lower outer periphery of the mandrel, which in the illustrative example is defined by the outer peripheral surfaces of the compression ring 25. It should be noted, should the powder during filling of the cavity and initial compaction of the powder therein by lowering of the plunger 10, filter out between the surfaces 38 and 39, imperfect compaction of the powder in the bottom of the cavity would result due to the dissipation of the pressure at the lower outer peripheral corner of the cavity. To prevent such escape of powder a destructi ble sealing means is provided. This sealing means comprises two portions which are bonded together and one of which is connected to the female die at the bottom of the die for downward movement therewith and one of which is supported by the mandrel and constrained thereby from downward movement with the female die. The bond between these portions is sufficient to withstand the pressure exerted by the plunger during initial compaction of the powder by the plunger. However, it is capable of being sheared in two at a higher pressure which is exerted on the female die during subsequent compaction by the dies and plunger concurrently.

In the form illustrated, the sealing means is provided by the shear ring 40 which has an inner annular flange 41 which extends entirely across the upper surface of the compression ring 25, so as to engage the lower edge of the inner peripheral wall 32 of the container 30, and forms the bottom wall of the cavity. A tight seal at the inner peripheral corner of the bottom of the cavity is not essential, as is that at the outer bottom corner, inasmuch as the escape of any metal powder at this point will be prevented by the peripheral surface of the mandrel or filler material between it and the container, as will later be explained. However, a continuous weld of the flange 41 to the lower edge of the wall 32 is preferable, being desirable both for assembly and disassembly of the die assemblage.

The inner portion of the ring 40 forms one portion of the sealing means and the other margin of the flange 41 forms the other portion of the sealing means. These portions are integral and consequently bonded together and form a joint capable of withstanding the initial compacting pressure. When a container is used also, the ring 40 is provided with an internal peripheral shoulder 42, adjacent the outer periphery of the flange 41, to which the lower margin of the outer wall 31 of the container 30 is welded. The lower plate 15 is preferably cha-mfered, as indicated at 44, at its lower internal edge to provide clearance for the weld of the wall 31 to the ring 40 at the shoulder 42. The ring 40 is not attached to the compression ring 25 but merely rests in place on the top of the compression ring 25. The plate 15 is insulated by sheets of asbestos insulation 45, interposed between its surfaces and the surfaces of other structure juxtaposed thereagainst.

As illustrated in FIGS. 1 through 3 and FIG. 6, the interior and exterior peripheral walls of the container are spaced from the exterior and interior peripheral walls of the female die 8 and mandrel 7, respectively, throughout the major portion of their lengths. This provides an annular clearance space 46 between the wall 32 and the exterior wall of the mandrel 7, which space extends from near the top of the mandrel entirely to the bottom thereof, and connects with the space between the walls 26 and 27 of the compression ring 25 and plate 5. Likewise, it provides an annular clearance space 47 between the peripheral wall 31 of the container and inner peripheral wall 20 of the female die, which space extends from the top of the female die down to the ring 15. The spaces 46 and 47 are filled with back-up material such as Alundum or silicon carbide. The upper margin of the inner wall 32 of the container is juxtaposed against and welded at its upper edge to the upper margin of the outer peripheral wall of the plate 14. The upper edge of the outer peripheral wall 31 is welded to a suitable pressure plate 49 which is juxtaposed on the upper face of the plate 18 and closes the upper end of the space 47. As mentioned, the lower margin of the outer wall 31 is welded to the shear ring 40.

It is apparent that wth this structure, the powder 28 within the container 30 cannot leak out the joint between the outer wall of the cavity and bottom wall formed by the flange 41, and thus into the space between the inner wall 38 of the female die and the outer wall 39 of the mandrel.

As to leakage of the metal powder between the inner peripheral edge of the flange 41 and the lower edge of the inner wall 32 this, of course, is prevented by the welded joint between the inner periphery of the flange 41 and the lower edge of the wall 32. Even if no weld were provided at this point, exfiltration of the metal powder would be so limited as to be negligible by the Alundum in the space between the inner wall 32 and the mandrel 7.

In operation, it is necessary first to move the plunger downwardly into the open end of the container 30 for effecting settling and slight initial compaction of the metal powder. For forcing the plunger 10 downwardly, suitable coaxial pressure rings 55 and 56 are provided. These rings are supported by the upper surface of the plunger 10 and are forced downwardly 'by engagement .of the upper surface of the ring 56 by a header 57 engaged, in turn, by the force transmitting member 4. Upon admission of pressure to assemblage 3, the ram 2 forces the rings, and thereby the annular plunger 10, downwardly slowly, thus forcing the filter means 34 .downwardly in the upper end of the cavity defined by the container 30. This downward movement settles and initially compacts the powder slightly within the container. This operation is performed in the absence of heating or at a temperature below sintering, thus allowing the gases and air gradually to escape. When a predetermined initial compacting pressure on the powder is reached, for example, 100 p.s.i., a vacuum is drawn in the furnace and the compacting pressure continued. The vacuum is gradually increased until it is approximately 2000 microns, this degree of vacuum being reached in a minimum of about two hours.

The vacuum is maintained and the temperature of the furnace is then slowly raised to approximately 1000 C. in about 15 hours. The compacting pressure is then raised in increments of 50 p.s.i. each five minutes until a maximum of 900 p.s.i is reached. Before this pressure is reached, the plunger 10 has entered the cavity to its full depth, thereby allowing the ring 55 to bear on the plate 49. When the pressure reaches a predetermined amount, with the ring on the plate 49, the female die 8 and the plunger 10 begin moving downwardly together.

It is to be noted that the upper end of the inner wall 32 of the container, as mentioned, is welded fixedly to the ring 14, as indicated at 48, so that the wall 32 cannot move downwardly and wrinkle. On the other hand, the outer wall 31 is securely welded, to the shoulder 42 of the shear ring 40 so that it is constrained to pull downwardly therewith, and is thereby prevented from wrinkling. As soon as the downward pressure on the female die 8 is sufficient, for example 500 p.s.i., the seal closing the outer peripheral corner of the die cavity is sheared, leaving the flange 41 supported on the compression ring 25. This seal, in the form illustrated, is the juncture of the flange 41 with the shear ring 40.

The effect of frictional binding of the powder is somewhat reduced because the outer wall 31 of the container 30 is held under tension and the frusto-conical portion of its surface faces somewhat downwardly toward the upwardly facing frusto-conical portion of the mandrel and moves downwardly with the powder to a great extent. This downward motion of the female die 8 continues until the final forming position is reached, as indicated in FIG. 3, this final position being determined by the shape of the annular elongated body to be formed and the density desired.

As a result of this initial settling and slight compaction while the mandrel 7 and the female die 8 remain in fixed axial position, followed by continued compaction imposed by the plunger 10 and augmented by the downward movement of the female die 8 relative to the mandrel 7, elongated hollow sleeve shaped and cup shaped bodies can be obtained. These bodies, in the as formed condition, have almost theoretical density, are free from contaminating lubricants, and have a length greater than twice the wall thickness.

However, when forming the bodies of beryllium, the temperatures used are such that quite often the highly heated mandrel, and sometimes the female die, exceed their elastic limit at the elevated temperature, and become distorted by the forming pressures at such relatively high temperatures. Under such conditions ordinarily it is necessary for the female die and mandrel to be replaced after each use if the next succeeding body is to be identical with the one first formed. It is to eliminate this replacement that the Alundum is provided in the spaces 46 and 47, in that substantial distortion in the die and mandrel can be compensated for by filling the spaces 46 and 47 between the next container 30 and the die and mandrel with the Alundum or other such back-up material. Of course, if the container substantially fits the mandrel and the female die without back-up material, the back-up material may be omitted, but generally the container cannot be so accurately fitted and, in any event, the back-up material assures ease in separation of the walls 31 and 32 of the container 30 from the female die 8 and the mandrel 7.

In those instances in which the metal powder can be formed at lower temperature and is of such a nature that it does not adhere objectionably to the forming faces of the mandrel and female die, the container 30 can be omitted. However, in such instances it is still necessary to provide a seal between the bottom wall of the cavity 9 and the bottom margin of the inner wall of the female die 8, for preventing the loss of material at the juncture of the lower inner peripheral margin of the female die 8 and the outer peripheral margin of the mandrel 7. This, too, can be accomplished by means of the shear ring 40 and flange 41. When the container 30 is not employed, an asbestos annulus may be disposed in the space between the wall 27 of the plate and the wall 26 of the compression ring to prevent the metal powder from entering this space and bonding to the ring 25.

With the powder thus prevented from escaping, effective initial compaction can be obtained which would not be obtainable were the powder permitted to escape at the outer lower corner of the cavity during initial compaction.

In order to charge the dies for forming a body, the following procedure may be employed.

The inner wall 32 of the container is first rigidly secured to the inner periphery of the annular flange 41 of the shear ring 40, preferably by a continuous weld. The outer wall 31 of the container 30 is next placed in position with its lower edge resting on the upper face of the flange 41 at the margin adjacent the juncture of the flange with the main body of the shear ring 40, and fitting against inwardly facing annular shoulder 42; provided at the juncture. This lower margin of the wall 31 and juxtaposed shoulder 42 of the ring are welded together, preferably by a continuous weld. The compression ring 25 is then placed on the surface 24 of the plate 5 in coaxial relation to the mandrel 7, in which position its inner peripheral surface 26 is spaced outwardly from the outer peripheral wall 27 of the plate 5. The assemblage of the container 30 and the shear ring 40 is then laid in position with the flange 4-1 lying on the upper surface of the compression ring 25, and with the inner peripheral surface 38 of the shear ring radially aligned with the outer peripheral surface 39 of the ring 25 and spaced therefrom with slight operating clearance. Next the plate 15 is positioned on the retainer 40 with the sheets of insulation in place. The plates 16, 17 and 18 are then successively stacked in place and the ring 40, plate 15, and superposed plates 16, 17 and 18 are bolted together. The refractory powder, such as Alundum or silicon carbide, is then placed in the spaces 46 and 47 and the assemblage vibrated to assure complete filling of the spaces 46 and 47. Next the top plate 14 of the mandrel is positioned in place and welded at its upper margin to the upper edge of the inner wall 32, as indicated at 48. The plate 49 is then laid on the top of the plate 18 with its inner margin overhanging and closing the upper end of the clearance space 47, which now has been filled, and is then welded to the upper edge of the outer wall 31 of the container.

Next, the metal powder, such as beryllium powder, is loaded into the container until the container is approximately filled, usually to about of an inch of the top. The assemblage thus far described is vibrated during the metal powder filling operation so as to assure effective settling and preliminary packing of the metal powder and the elimination of as many air pockets as possible in the charge of powder. Next the filter means 34 is placed on the top of the powder and spans the space between the inner and outer walls of the container 30 and is in wiping contact therewith so as to permit the escape of air and gases through the filter while preventing the escape of powder therewith.

Next the annular plunger 10 is placed on top of the filter means 34. The annuilar plunger 10 is particularly desirable in that it prevents the formation of low density areas which would otherwise be present around the upper end of the charge. There is always a possibility of such improper compaction at this location as a result of a void in the 'body becoming filled by powder so that the void migrates to the top of the compact prior to and during compaction, thereby causing a powder slump. Powder slumps may result from a number of causes. For example, when the loaded assemblage is placed in the furnace, the powder level is at its maximum vibrated height, but as a vacuum is drawn in the furance,

and therefore on the powder, there is an additional settling of the powder because of the removal of air and gases. Further settling of the powder occurs as the compact is brought up to si-ntering temperature. As such a slump occurs, the filter means 34 displaces the powder and causes it to fill the voided area until the entire bottom surface of the filter means 34 is juxtaposed on the upper surface of the charge and held firmly by the plunger 10, thereby assuring equalized pressurization throughout the compact and, in turn, uniform densification. With the die assemblage arranged as thus described it is sintered while subjected to the ram pressure, as heretofore described.

Thermocouples may be provided in suitable bores in the female die and mandrel, as desired. For example, bores 50 and 51 may be provided in the plates 17 and 18 of the female die, respectively, for receiving thermocouples.

At this point it is to be noted that the pressure at which the sealed joint, in the outer bottom corner of the cavity, is broken by shearing of the flange 41 can be effectively controlled by the provision of a peripheral slot 52 at the juncture of the flange 41 and the main body of the shear ring 40. This slot not only assures a sharp break upon shearing to provide a relatively smooth edge but also, depending upon the amount of metal removed, predetermines the pressure at which shearing will occur, which, in the illustrative example, is a pressure of about 500 psi. on the material.

After the shearing occurs, thus destroying the seal at the lower outer corner of the cavity, the pressure continues to be incrementally increased until that for a preselected calculated compaction and densification of the sintered material is reached. When this maximum densification has been reached, pressure is reduced and held at a lower level until six successive pressure readings at the lower level, taken at five minute intervals, have been taken with no axial movement of the female die occurring. After reaching the required compaction, the press is shut off, and an inert gaseous medium, such as an argon gas, is introduced into the furnace. No soaking period is required. As soon as the temperature recorded .by the thermocouples falls below 1,000 C., the assemblage is removed from the furnace and the mandrel, except for the compression ring 25, is stripped therefrom. The remaining portion of the assemblage is then permitted to cool to ambient temperature after which the beryllium body is removed from the female die.

It is noted the flange 41 is maintained in spaced relationship to the remainder of the mandrel assembly by the 'ring 2 5. This facilitates removal of the formed body from the mandrel assembly. The reason is that during the completion of the forming stroke, a small portion of the heated metal is extruded into the clearance space between the peripheral surface 39 of the ring 25 and the inner peripheral surface of the cavity formed by the outer wall 31 of the container 30, thereby creating a metal-to metal bond between the ring 25 and the wall 31. Consequently, upon removal of the mandrel, the ring 25 is lifted off the plate 5. The plate 14 is lifted with the remainder of the assemblage because the plate 14 is welded to the inner wall 32. If the ring 25 were not provided, it is apparent that the base plate 5 would be bonded to the outer assemblage, and removal of the plate would be expensive.

It is to be noted that the outer peripheral surface of the ring 39 is cylindrical at its upper margin where it is aligned with the inner peripheral wall 38 of the shear ring 40. For a short distance therebeneath, the wall is frusto-conical to provide an undercut so that the sheared surface of the ring 40 can move readily therepast without binding. This limits the metal-to-metal contact between the shear ring 40 and the ring 25 to a very short distance axially of the ring 2 5.

In the form illustrated in FIG. 1, wherein the powder 9 charge before compaction is about 23 inches long and has an inner diameter at the base of about 29 inches and an outer diameter of about 32 inches, the predetermined calculated compaction is obtained by about inches of downward movement of the female die. The pressure during sintering is about 900 p.s.i. and is subsequently reduced to about 750 p.s.i. with a temperature at about 1000 C. Due to the frusto-conical shape of the portion of the container wall 3 1 overlying the wall and adjoining pants of walls 19 and 2 1, its overhanging relation to that of the frusto-conical portion of the container wall 32 overlying the outer peripheral walls of the plates 1-1 and 12, these portions of the container wallls approach each other during sintering, consequent upon downward movement of the female die, and thus assist in distributing the applied pressure more effectively and in reducing friction. The value in the above example will be varied, of course, with the size and shape of the body, the degree of densification required, and the particular metal used.

For ease of disassembling, thin asbestos rings 53, approximately /s of an inch thick, may be interposed between the surface 24- and the under face of the ring 25, and between the flange 41 and the upper face of the ring and the lower face of the ring 24. Like asbestos rings 54 and 54a may be disposed between the top face of the plate 18 and the annular plate 49, and between the mandrel plate 13 and the top mandrel plate 14, respectively. The use of asbestos in these places is generally to reduce or prevent any tendency toward pressure welding.

In FIGS. 7 and 8, a modification of the preferred embodiment is illustrated. It employs a female die 70, mandrel 71, and container '72 to form a body 73. It differs from the preferred form primarily in that no frusto-conical portions are provided on the female die 70, and the taper on the mandrel 71 is reduced so that it provides only a slight draft for facilitating the removal of the container 72 and enclosed body 73 during the stripping operation. In this modification, the incremental increase is 100 p.s.i. every five minutes, instead of 50 p.s.i., and the maximum pressure is 1600 p.s.i., instead of 900 p.s.i. An axial movement of 17 inches is used to obtain maximum densification, and the reduced pressure is 1400 p.s.i. instead of 750 p.s.i. The time and temperature values remained unchanged.

The body has a length of about 23 inches, with a maximum outside diameter, at the base, of 32 inches, and an inside diameter at the base of 28 inches, tapering to about 27 inches at the top.

As mentioned, with some metals, the apparatus as shown in FIG. 2 may be modified by omitting the outer wall 31, the inner wall 32, and compression ring 25, or any one of them, without departing from the scope of the invention.

For convenience in illustration, the die assembly has been described in an operating position in which the common axis of the female die and mandrel is vertical and in which the relative forming stroke of the female die and mandrel is effected by downward movement of the female die. It is apparent from the illustrative example, however, that the assemblage may be disposed with its axis in other positions, and the operating stroke may be eifected by moving the mandrel toward the die or by movement of both the mandrel and die toward each other.

Although the above description and illustration are of a detailed character, it is to be understood that changes and modifications may be resorted to without departing from the spirit of the invention.

Having thus described our invention, we claim:

1. A method of forming metal powder into elongated hollow bodies, said method comprising confining a charge of metal powder to be formed in an annular cavity of predetermined shape sealed at one end, applying mechanical pressure to the other end of the charge in a direction toward the one end while holding the opposite end in fixed position for preliminarily compacting the powder, while completely confining the charge in the cavity at said one end, subjecting the powder to a sub-atmospheric pressure during said preliminary compaction, gradually heating the powder to a temperature up to the sintering range while maintaining said preliminary compaction and sub-atmospheric pressure, and then, when the powder reaches the sintering temperature range, gradually increasing the temperature and increasing said mechanical pres sure on said other end of the charge, and concurrently changing the size and shape of the cavity and breaking the seal for compacting the sintering material to a predetermined sintered density while maintaining said subatmospheric pressure.

2. The method according to claim 1 wherein said change in size and shape is by changing both the width and length of the cavity concurrently.

No references cited.

LEON D. ROSDOL, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner. 

1. A METHOD OF FORMING METAL POWDER INTO ELONGATED HOLLOW BODIES, SAID METHOD COMPRISING CONFINING A CHARGE OF METAL POWDER TO BE FORMED IN AN ANNULAR CAVITY OF PREDETERMINED SHAPE SEALED AT ONE END, APPLYING MECHANICAL PRESSURE TO THE OTHER END OF THE CHARGE IN A DIRECTION TOWARD THE ONE END WHILE HOLDING THE OPPOSITE END IN FIXED POSITION FOR PRELIMINARILY COMPACTING THE POWDER, WHILE COMPLETELY CONFINING THE CHARGE IN THE CAVITY AT SAID ONE END, SUBJECTING THE POWDER TO A SUB-ATMOSPHERIC PRESSURE DURING SAID PRELIMINARY COMPACTION, GRADUALLY HEATING THE POWDER TO A TEMPERATURE UP TO THE SINTERING RANGE WHILE MAINTAINING SAID PRELIMINARY COMPACTION AND SUB-ATMOSPHIERIC PRESSURE, AND THE, WHEN THE POWDER REACHES THE SINTERING TEMPERATURE RANGE, GRADUALLY INCREASING THE TEMPERATURE AND INCREASING SAID MECHANICAL PRESSURE ON SAID OTHER END OF THE CHARGE, AND CONCURRENTLY CHANGING THE SIZE AND SHAPE OF THE CAVITY AND BREAKING THE SEAL FOR COMPACTING THE SINTERING MATERIAL TO A PREDETERMINED SINTERED DENSITY WHILE MAINTAINING SAID SUBATMOSPHERIC PRESSURE. 